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Which sub-scale setting may a pilot use on an instrument approach in South Africa--QFE or QNH?
I saw this question in my Instrument Rating exam and I need to know which answer is correct, A or B.
Question: As far as the vertical reference of an aircraft during the final approach segment of an instrument approach procedure is concerned, a pilot:
A. may only use the local QNH setting on the altimeter sub-scale
B. may use the altimeter with QFE on the sub-scale
So far I have looked in ICAO 8168 and the SA CATS and CARS 2011 and this is what I found:

This excerpt is from the SACAA CATS and CARS documents:

8 Approach and Landing
8.1 A QNH altimeter setting shall be made available to aircraft in approach clearances and in clearances to enter the traffic circuit.
8.2 A QFE altimeter setting, clearly identified as such, should be made available in approach and landing clearances on request.
8.3 The vertical positioning of aircraft during approach shall be controlled by reference to flight levels until reaching the transition level below which vertical positioning shall be reference to altitudes, except as provided for in paragraph 8.4.
Note: This does not preclude a pilot from using a QFE setting for terrain clearance purposes during the final approach to the runway.

Please bear in mind that many of the SACAA exam questions are ambiguous or rather test your understanding of grammar and semantics.

I have never been given a QFE to conduct an instrument approach in S.A. It does happen in some countries but then everyone has to use it. If you look on an approach chart you will see the vertical profile is given in terms of height above sea level so you wouldn't be able to conduct the approach with QFE set. Edit: The SACAA charts do show height of the waypoints of the vertical profile, but Jeppesen doesn't. The MDA and DH are given terms of elevation and height, but this is to use in conjunction with the radio altimeter. QFE might be used by aerobatic pilots or some military operations but I think its use is rare. Having said that, the AIP does appear to allow the use of QFE on a second altimeter during the final approach see note(2) and point (f) below. Therefore it appears that B is the answer the CAA is looking for. Using the word "only" in answer A would make it incorrect.
Please look at the AIP here with particular attention to the Altimeter Setting Procedures, I will try elaborate in due course.
Firstly the AIP says

The altimeter setting procedures in use conform to those contained in
ICAO Doc 4444 ATM/501 Procedures for Air Navigation Services and ICAO
Doc 8168 OPS/611 Procedures for Air Navigation Services - Aircraft
Operations

Then it says

During flight in the vicinity of an aerodrome at or below a fixed
altitude called the transition altitude, an aircraft shall be flown at
altitudes determined from an altimeter set to sea level pressure (QNH)
and its vertical position shall be expressed in terms of altitude.

Further down below it says

On reaching the transition level at least one altimeter within the
aircraft shall be set to the QNH and thereafter the vertical
positioning of such aircraft shall be by reference to altitudes.
Note 1: On reaching the transition level, pilots shall reset their
altimeters to the QNH without requesting Air Traffic Control (ATC)
permission to do so and without notifying Air Traffic Control (ATC)
that the change has been made.
Note 2: This does not preclude a pilot from using a QFE setting for
terrain clearance purposes during the final approach to the runway.
f)   During the approach to land, terrain clearance may be determined
by using either a QNH altimeter setting (giving altitude) or a QFE
setting (giving height above the QFE datum)

I believe the intention is to allow the pilot the discretion of using a second altimeter with the QFE set, but when you talk to ATC it will be with reference to the altimeter having QNH set.

Thank you Raffles for your detailed response. I also thought that QNH was the only vertical reference to be used on instrument approaches. However, in the process of researching this question, I recalled that radio altimeters use QFE on CAT II and III approaches. Since the question doesn't specify a specific instrument approach and is referring to any instrument approach, I thought that it might rule out "may use only QNH" as an answer and make answer B the more correct answer. What do you think?

Hi Student_Pilot, I'm beginning to think the same, due to the wording of the two answers. I put the question on Avcom and some guys also think the same. I looked in ICAO document 4444 and the wording is the same as the AIP (so the CAA copied 4444). I will modify my answer shortly.


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Could liquid alkali metal fuels be used for ramjet/scramjet propulsion?
In particular, I'm thinking about lithium, but sodium could be considered as well.
Lithium has low molecular weight, very high propellant density, and low melting point. At sufficiently high temperatures (above 500 degrees Fahrenheit) it ignites spontaneously in air with explosive force.
A lithium scramjet/ramjet would require an onboard power supply to heat the fuel to liquid and then pump it into the combustion chamber. The idea is that the combustion chamber would need to be hot enough so that the lithium droplets would quickly evaporate and ignite making the mixture hypergolic.
I'm surprised that I have found no current R&D into alkali metal fuels for rocket propulsion(let alone hypersonic propulsion). It seems to me that to make longer range hypersonic flight possible, exotic fuels will be absolutely necessary.

Energy per unit weight pf lithium is actually half of jet fuel. You might have better luck with aluminum. Technically it's not too difficult since solid ramjet is already done, you add metal into the base fuel.

Hydrogen has such a high energy density than anything else that hydro carbon is almost next-to-ideal, which is why kerosene and methane are both considered very good rocket fuels.

@user3528438 But it has a very low physical density. Which is why it is impractical for usage as a fuel because of the weight and volume of a sufficiently large fuel tank. Liquid H2 is cryogenic which has its own set of problems as seen by the Space Shuttle.

@user3528438: For example, the Space Shuttle solid rocket boosters, which used aluminum metal with an ammonium perchlorate oxidizer.

How long does the lithium take to evaporate and then to react with oxygen? Air in a scramjet is moving extremely fast (supersonic by definition!), so there is very little time left for mixing and combustion if you desire to create thrust from the the combustion heat energy.
Your lithium engine will most likely trail a bright contrail of burning lithium but develop rather little thrust. The high flame speed of a mixture of hydrogen and oxygen makes hydrogen the preferred scramjet fuel.

In that case, it would be better to make it a ramjet where the combustion chamber temperature is > 2000 degrees Fahrenheit. Hydrogen has low density and this makes it not very practical for use in a ramjet/scramjet. So for using something like lithium this would require subsonic, compressed, heater air and possibly  spark plugs to rapidly vaporize the lithium fuel droplets and ignite them.

@Mr X your thinking is good, just consider Hydrogen as a metal!  Now check the electrons available for chemical combustion vs weight.  Liquifying it helps the density issue.  But they do use aluminum in solid fuel mixtures rather effectively.

@RobertDiGiovanni I certainly do. But liquid hydrogen is cryogenic which means that the tank needs insulation which adds to size and weight. Lithium is much more reactive than aluminum.

There are some good charts on Google about Mj energy per kg of various elements and also energy density.  What PK said about mixing and burn time in a very short time frame factors in, no problem vaporizing liquid H2. Cryrogenics can also be used to help cool the combustion chamber before injection.  But a lithium solid booster is interesting.

Of course, if you burn the fuel using a detonation wave (which has a speed a couple orders of magnitude greater than that of a wimpy flame front)...

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What is the significance of the speed of sound to aerodynamics and aircraft design?
Almost every aspect of flight seems to be affected by the speed of sound, somewhat in the same way that nearly everything in physics is determined by the speed of light.
Obviously it's not exactly the same - it is possible to travel faster than the speed of sound, unlike the speed of light - but still, what happens below the speed of sound is quite different from what happens above the speed of sound, while the region close to the speed of sound seems to be a mysterious transition zone.
This isn't just limited to aerodynamics, but also for example to the way the fuel/air mixture burns in a jet engine.
So, why is the speed of sound so significant for aerodynamics and aircraft design?
I presume that it is something to do with a limit on how fast information about the movement (or pressure changes) of air molecules (i.e cause and effect involving air molecules) can travel through a medium of those molecules, and that "speed of sound" is shorthand for that speed.
These movements of air molecules presumably also might be in a form that we wouldn't ever actually hear (frequency too low, frequency too high) or indeed that we wouldn't ever experience as a sound (a single pulse rather than a vibration); in this case "speed of sound" seems to be a rather misleading way of describing it.
I assume that there are analogous issues around the speed of information of pressure changes in water, so that hydrodynamics has to deal with them too. I imagine that because is much less compressible than air, that the speed would be much higher, so perhaps less likely to trouble hydrodynamic designers and engineers.
I think my high-school physics seems to be more or less enough to deal with these concepts, but the language around speed of sound does seem to hide away some of the key ideas, and leads my intuitive grasp in the wrong directions.

There is a lot of friction heat at those supersonic speeds.

most of the rules of aerodynamics change when going supersonic

@JonathanWalters I'm not keen on the edit narrowing the question to "aerodynamics and aircraft design". Of course I'm asking it from the point of view of aviation, but what I am trying to understand is the physics of the speed of sound, rather than just aircraft design or aerodynamics. Hence my parallel with the speed of light, which in physics represents a vast range of conceptual and actual constraints, and can be expressed as a limit on the speed of transmission of information. Similarly, I'm concerned with other implications of the speed of sound (for example the combustion of gases).

@DanieleProcida My edit was an attempt to keep this question from being closed as off-topic for Aviation.SE. If you want answers to the broader physics question you might find that you need to ask that question on Physics.SE. Feel free to roll back my edit, but I fear your question may be closed if it is too broad.

Additionaly, if you would prefer to have your question migrated to Physics.SE a mod can accomplish that for you.

Note that it is. in fact, perfectly possible to move faster than light.  (The trick is to slow the light down.)

Your question shows that you know that the speed of sound is the speed of small pressure changes in a fluid, and moving faster than that will give the fluid ahead no warning of an approaching disturbance. I will, therefore, dig a little deeper in my answer, fully knowing that I can only scratch the surface. I encourage you to ask more specifically if one topic piques your interest but is not sufficiently explained.
Short answer
The speed of sound is significant in aircraft design mainly in:

Splitting the flight regimes into two areas with markedly different flow behavior,
helping to convert a geometry for compressible flow to one which can be calculated by simpler, incompressible methods,
limiting the maximum practical flight speed for efficient air travel and
giving the designer hints how to push this boundary up.

In the combustion of gasses the speed of sound is of less significance than the speed at which a flame advances. This is more complex (PDF!) and depends on the turbulence level, mixing ratio, temperature and the activation energy for the combustion reaction to start.
Explanation
To explain the influence of the speed of sound let me look at the molecular level: Air is a collection of molecules which zip around constantly, bumping into each other frequently. We express this with macroscopic parameters:

The intensity of the collisions is measured as pressure,
the number of molecules per volume is measured as density, and
the speed of the molecular movement is measured as temperature.

At rest, the frequency and intensity of these collisions is the same in all directions. If something moves through the gas at a speed far below the speed of sound, the collisions are still roughly of the same intensity all around. The local intensity may vary, but the molecules will stay equally distributed over the volume. This means that the disturbance will cause a change in pressure, but density will be unaffected.
Similarity methods
Moving faster, the disturbance will cause more intense collisions in the direction of movement and will start to push the molecules together: Now it will also cause weak density changes, and those grow with the square of of its speed. To regain an equal distribution over space, the molecules will now move sideways (and up and down), and the pressure changes will reach farther out to the sides. When you compare the pressure fields at different speeds, the "faster" field will look like one made by a more narrow disturbance at lower speed. This effect is captured by the Prandtl-Glauert factor $(1-Mach)^2$. Scaling the Y- and Z-coordinates of an aircraft geometry (where X is in flight direction) with this factor will result in a geometry which can be calculated with incompressible methods.
At the speed of sound this factor has a singularity, so its application should be restricted to speeds below Mach 0.7. Above Mach 1 a similar transformation can be applied, now using the Ackeret factor $(Mach-1)^2$.
Convergent and divergent flow
The ratio of density changes to pressure changes continues to increase such that in supersonic flow the density changes become dominant. This shows when looking at the flow in an expanding or contracting duct:

Subsonic flow in an expanding duct will slow down to preserve density and will accelerate in a contracting duct.
Supersonic flow in an expanding duct will accelerate further, reducing density and pressure, and decelerate in a contracting duct.

This different behavior is used when accelerating the compressed gasses flowing from an aircraft turbine: First the nozzle diameter is reduced until the gas accelerates to the speed of sound. To accelerate the gas further, now the diameter has to increase again in flow direction. The result is a convergent-divergent nozzle, a typical part of supersonic jet engines.
Wave drag
Now picture the aircraft in slow motion from the side as it slices through the air at supersonic speed: Since pressure increases while the aircraft body expands and decreases once it contracts, you will have two drag components in supersonic flow which do not exist in subsonic flow: Forward facing parts experience higher pressure and rearward-facing parts experience suction. This effect is called wave drag and makes supersonic flight inherently less efficient that subsonic flight.
Transsonic design
The same effect also affects the pressure field around an aircraft which is moving close to the speed of sound. Its movement displaces air and has an effect similar to a contracting duct, accelerating the flow in its vicinity as long as the aircraft grows thicker in flow direction, and decelerating it again over its rear half where it contracts. Once this local flow reaches the speed of sound, it will accelerate further while the aircraft contour contracts, leading to a supersonic pocket of air which will not slow down. It does so eventually, collapsing in a local shock which can cause flow separation and high local drag. Even worse, this separation will affect the whole pressure field and reduce the speed increase, which opens a feedback loop which will make the shock fluctuate. This oscillating shock can excite vibrations, render control surfaces ineffective and can incite flutter.
To reduce the local speed increase the surfaces of airplanes can be swept so the flight speed can be increased without running into the aforementioned transsonic effects. Also, sweep will smear out the sonic effects when accelerating through Mach 1, so the drag peak around Mach 1 can be greatly reduced.
Around Mach 1 it is important to limit the rate at which the aircraft's body expands or contracts over the direction of flight, and the wider-ranging nature of pressure changes allows the designer to expand one part of the aircraft to compensate for the local contraction at a different part when both are at the same position relative to the direction of flight. The consequence of this is the area rule which shows in coke-bottle fuselages or big flap track fairings.

An excellent answer for the math challenged among us! Thanks Peter!

@FreeMan: You are very welcome! I enjoy answering this type of question, you know ...

Gasp! I'd never noticed!!  ;)

You say the intensity of the collisions is measured as pressure.  Would it be accurate to say the "momentum" of the collisions is measured as pressure?  I've always thought of pressure as being analogous to density, but I trust your expertise more than mine.

@PJNoes: Pressure and density are tied with volume and temperature in the ideal gas law. When measuring, you measure force per area, and this could very well be the integral of the momentum of the collisions, but I don't know how to deal with time in this context. I only use the picture of colliding molecules because it helps me to visualize why things are as they are. I did not bother to go all the way to a mathematically correct explanation. I guess this should be asked over on Physics SE.

This is really an amazing answer! Thanks Peter!

You're correct that the speed of sound is important because it has something to do with how 'information' is transmitted through the gas.
The 'speed' of sound is actually the speed of transmission of a (small) disturbance through the medium. Air is a compressible medium, in general. When there is an infinestimal pressure change generated at some point of the flow due to some disturbance (say, an airfoil moving), this will be propogated throughout the air as a pressure wave at the speed of sound.
Disturbances are propogated in the air due to collisions involving (randomly moving) molecules. Due to this, the air molecules in the path of the disturbance 'knows' that there is something coming on their way and try to get out. Consider the following image of an airfoil in a smoke tunnel.

Image from forums.x-plane.org; credited to Alexander Lippisch
Here, you can see that the air (made visible through smoke) has already 'moved' out of way of the airfoil before coming into physical contact with it; this is because of the disturbance, which was propogated through the air at the speed of sound.
Now, if the flow velocity exceeds the speed of propogation of these disturbances, they will 'pile up' to form strong (pressure) waves, which we call shock waves. The following image shows a model of the X-15 in a wind tunnel.

By NASA - Great Images in NASA Description, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6450008
The disturbances are still being propogated; but the flow velocity is greater than that- the result is that the molecules have no way of 'knowing' that there is an object is on the way and pile up over it.

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What happens when VFR conditions change over a longer flight?
If I am understanding correctly, you're not really supposed to use VFRs if the weather is not clear enough that you can ensure you're not going to run into things. Usually you do this by looking at the weather reports around your route and going "nice and sunny, VFR should be fine". Then picking an altitude that meets your other needs, but keeps you out the clouds (if there are clouds).
Then, because you are flying VFR, when you're outside of certain air spaces, you're "on your own". No ATC, or things like that. Maybe some flight following, where the ATC guys goes "Ummm, you see that big mountain right? you're not going to hit that are you?" But also no guarantee of that either (because you're supposed to be using your eye balls to see that you don't smack into the mountain).
But what happens when conditions change, either because the flight is taking longer than expected, or because they just do, and stuff happens? You seem to be in a rough spot. You should not continue your flight using VFR, but you can't land. So what do you do, specially if not rated for IFR flight?
Specifically, conditions are no longer acceptable for VFR flight, and you're not really near your destination, and the smaller airports around you are not currently staffed.

I see no Question in your question.  (you put a question mark in your title, but just barely)

Hmm I will try to rephrase a bit.

See Why does flying IFR require a rating? and What constitutes an “emergency” in flight procedures?

See What happens when a pilot has no Instrument Rating and visibility drops?

Did you just vote to Dupe your own question?   Just delete it.

You wrote:
"you should not continue your flight using VFR, but you can't land"
"and the smaller airports around you are not currently staffed."
Why can't you land?   If you can't land, you should not be flying!
Why do you need an airport to be staffed to land there?
Pilots land at unstaffed airports all the time following CTAF (Common Traffic Advisory Frequency) procedures.
If weather conditions are deteriorating below VFR minimums, then landing at an airport is 100% an appropriate course of action.

I think I may have over though it a bit. I was meaning like "all of the sudden you can't see the airport". But now, trying to express that, it's not like fog suddenly appears out of no where and you get no warning.  It would be a very strange (but I guess not impossible) occurrence to go from nice and well above VFR minimums to "wow I can't see the ground" that quickly.

Unless you are along the coast, where water & land temperature differences can create fog very quickly.

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Would the Concorde have been better off using a tailed delta wing?
In one of the answers to this question about the Concorde's lack of high-lift devices on its wings, it was pointed out that the Concorde, using a tailless delta wing, wouldn't have been able to use flaps, since there was no horizontal tail aft of the wing to counteract the downward pitching moment that would result from deploying flaps.
On the other hand, a tailed-delta configuration (a delta wing with a conventional tail mounted aft of the wing) would have been able to use flaps, as it would have had a separate elevator further aft with the ability to counteract the flap-generated pitching moment.  A good example of this would be the design for the Boeing 2707-300, which was to use a tailled-delta configuration and would have had some rather impressive flaps, but was cancelled in 1971 before even the two prototypes had been completed.
Given that it would have allowed the inclusion of flaps, would a tailed-delta configuration have been a better choice for the Concorde than the tailless delta they ultimately went with?
Edit: Better in that it would have greatly increased the Concorde's subsonic efficiency and allowed lower-speed (and thus less potentially-tyre-explodey*) takeoffs.
Quoth the linked question:

...The lack of high-lift devices considerably reduced lift at low speeds, requiring the Concorde to take off and land at higher speeds and angles of attack than if it had been equipped with flaps and slats. As well as greatly increasing drag (and thus fuel consumption) during takeoff and landing, this greatly increased the force carried by the main landing gear and the speed at which its tyres had to spin; as a result, if and when a tyre blew out (due to, for instance, a piece of debris left by the previous airplane to use the runway), the results would be much worse than with a tyre blowout on a subsonic airliner.
...[The addition of high-lift devices], along with slightly larger wings and more powerful engines, would have allowed the Concorde B to dispense with afterburners, considerably increasing its fuel efficiency and range.

* It happened a lot before the fatal accident.

A canard would be way better. At that moment canard wasn't used probably due the the lack of well tested fly-by-wire system to counter the instability introduced by a front (de)stabilizer. If we were to redo the design today, or did a another more risk-taking design at that time(XB70, Tu-144), then probably that's the way to go.

@user3528438, canard configuration can be just as stable as the tail one and tail one can be just as unstable as canard. After all, F-16, the first fighter designed to be unstable, has conventional tail. The only difference is that for stable configurations tail is more efficient and for unstable ones, canard is (as far as I can tell, both XB-70 and Tu-144 were stable).

@user3528438, also remember that Concorde was supercruising at M2, which is way better than any fighter today. At M2, the shock cone is 60° and for efficiency, the aircraft should fit within that. That does not leave much space to put the canards in.

The beauty of the pure delta configuration was you could get a reasonably low supersonic drag configuration in a planform with a lot of wing area, essential for really high altitude cruise and for reasonable landing speeds without high lift devices.  It was a good overall compromise for Concorde's mission.
The Avro Arrow, produced then scrapped in Canada in the late 50s, also used a large pure delta plan form (which is thought to have had some influence on Concorde, a lot of Avro's engineers having joined the Concorde program when Avro Canada vanished).  This allowed it to meet the RCAF's specification that required M2+ while being capable of sustaining a 2G turn at 50,000 feet, something no interceptor could do at the time.


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How does the adiabatic cooling of wingtip vortices work?
This question on condensation and wingtip vortices, What is the mechanism by which condensation forms above wings?, attributes the reduction in temperature found in wingtip vortices to Adiabatic cooling.
The reasoning for this, as quoted from Wikipedia, is given as:

Adiabatic cooling occurs when the pressure on an adiabatically
  isolated system is decreased, allowing it to expand, thus causing it
  to do work on its surroundings. When the pressure applied on a
  parcel of air is reduced, the air in the parcel is allowed to expand;
  as the volume increases, the temperature falls as internal energy
  decreases

Emphasis mine
My question is: why are we assuming that a parcel of air—which, according to Bernoulli's principle has  a lower internal pressure—would be allowed to expand against the external pressure which would be higher?

This reasoning to me would suggest that the parcel would compress (if it was compressible) and that its temperature would increase.
Also, why are we assuming that air is now compressible below Mach?
Wouldn't it make more sense to attribute the reduction in temperature to an isochoric cooling process?  This would only require the internal pressure to decrease, and would not require a change in density.

@frederico I have asked a follow up question to your answer , hope you can help

The vortex is rotating. This by itself should suffice to explain the expansion at its core. The mechanism is less Bernoulli (which applies to straight flows) but centrifugal force. Compare it to the decreasing height at the center of the bathtub vortex when water flows out the drain and starts to rotate. The height is proportional to the pressure in the fluid. The same happens with air - the pressure change is highest at the core because the angular velocity decreases with the distance from the core.
And, yes, air is compressible below Mach 1. It is only incompressible ideally and at Mach 0. The density changes are minuscule at low speed, but they are there. The thermodynamic process is an adiabatic one (no heat flow) with changes to pressure, volume and density (a tiny bit).

Thanks for your very good reply Peter , a few questions I always understood that the radial force of the pressure gradient is what keeps the flow in uniform rotation against the inertial centrifugal force . Are you saying that in the core the centrifugal if greater than the centripetal and this is what allows the expansion? Also when we allow expansion in the core we must have also compression outside the core? So outside the core heats up as the core cools down?

@QuentinChester: The flow is in an equilibrium between centrifugal (= inertial) and centripetal (= pressure) forces. Only the expansion creates a centripetal force which balances the centrifugal force of the rotating flow. Technically, there is a compression outside, but it has a near-infinite volume to work on, so effectively only an expansion can be observed which dies down with increasing distance from the vortex core.

The air already inside the vortex is not expanding. It is already expanded and is being slowly recompressed again as it's kinetic energy is dissipated and the vortex slows. The expansion occurs when the air enters the vortex.
The oncoming air is being sucked into low-pressure pocket created by the wing and it expands to fill it. Afterwards it is just the centrifugal force of the rotation opposing the pressure gradient and preventing collapse of that low pressure pocket, but the pressure gradient is not increasing any more. It is slowly decreasing as internal friction slows the rotation.


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What's the purpose of airline CREW tags?
I see it sometimes on airline crew luggage, but not always. I see these come with airline name, sometimes airline base code and large CREW.
What is the purpose of these airline crew tags? After all, I got one in Crew Outfitters store as "collectible". Is this official airline tag which gives priority, different way of airport handling or something?
What would happen if I put it on my suitcase (either checked or carry-on).

It’s not an official/legal tag. Just a reminder for ground crew. At my company, luggage handlers unloading checked in luggage from the cargo will leave the bag near the aircraft (to be picked up by the crew) rather than bring it in to the terminal through the normal luggage flow. The ideea being that bag belongs to a crew member (active or positioning) and he might move arround the apron without ever going into the terminal.
Mind you this is just a courtesy, not a guaranteed thing. Should you put this on to your (checked in) bag there is a good posibility a ground operator will leave it under the plane for a while, then, when no crew comes to claim it, it will go through a lengthy process back to terminal. So your bag will have to honor of being one or two hours late to the conveyor belt.


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Is there a legend for figure 15 in the Airman Knowledge Test Supplement for Instructors?
Here's why I'm asking. While using King's free practice test tool, I ran into a question:

about figure 15: in the Instructor test supplement asking about the percentage values for the MID area. I would just memorize the answer, but that's not a thing anymore, so I'm trying to find out where I can get the actual information. It seems like convective outlooks and "TSTM" outlooks are different things, as I have found both on NOAA's website. AC 00-45H only talks about convective outlooks, and doesn't even tell you the percentage values. I found this on the website that breaks it down.
But, that's not for the chart in the test supplement. The closest thing I found on NOAA's website now just shows the percentage values directly on the chart, and they are very different from the answer choices on this particular question.
I hope there is a legend somewhere staring me in the face and I've just missed it!
P.S. King says the answer is A.

I think this is a case of the test material lagging behind reality.  I suspect, and hope, the next issuance of the test materials will have a revision to the Categorical Outlook.


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How do I know if a runway has REIL?
I'm designing an airport of a flight simulator. One of the runway settings is Runway End Identifier Lights (REIL). Is there a way I can see if a runway at a US airport has REIL via the internet, and whether they are Omnidirectional or Unidirectional REIL?

If the runway has other runway lights, it typically would have REILs. If the runway has an instrument approach, it typically has MIRL or HIRL and the REILs.

@Ksisky I am not aware of any database where you could find information on whether a REIL installation is Omni- or Uni-directional. The only way I know of is to go and see for yourself what they actually are.

You can find this information in the Airport Facility Directory which is published in print by the FAA and is available for free from them online here. REIL will be listed for the specific runway for the airport in question. For example I fly out of KDYL and if we take a look at the page in the AFD for KDYL you will see REIL listed for RWY 23

(source)


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What is the expected range of handheld radio?
For a handheld VHF radio carried as an emergency backup, how should I expect the effective radius of a handheld radio to compare with on-board radios in two-way communications? I figure reception should be the same. But transmission would probably be different, dependent on the power. Would the difference be significant? What factors would change the useful radius of a handheld radio?

The dominating factor in radio range calculation is antenna height over ground.

I think this question is fair game in Aviation.SE because it relates to good aeronautical decision making regarding safety equipment. It is no less applicable than http://aviation.stackexchange.com/questions/8580/how-do-you-calculate-the-maximum-distance-to-communicate-with-tower-control, which is not on hold.

I think this is an excellent question. Even if the answer is "depends on X, Y, and Z factors" it should lead to informative answers.

The range would depend on whether you are inside the aircraft . In the middle of the aircraft the range would be from the cockpit to the tail section. In the cockpit with the antenna near the window at 37,000 feet approx 300 miles line of sight with a major dead spot directly below or behind the aircraft. On the tarmac in the same position about 6-10 miles.

This ICOM hand held has a max output of 5W compared with their panel mount radio which outputs at 8W. Unfortunately they don't quote effective ranges on their website (and its highly dependent on many factors anyway). In any case you are limited by line of sight which depending on the plane you are flying (and how high up you are) may be more of an issue. There is some discussion here on the matter and generally people say the work pretty well. Keep in mind also that the aircrafts installed radios have their antenna that are mounted on the exterior of the plane which has an effect on the reception and transmission ability. Using a hand held inside a plane may in practice yield different results even if it has the same transmission specs as an installed radio.

With that small of a power difference antenna location will have a significant effect on transmission range. If the surface of your craft is metal that will act as an attenuator for an internal transmitter and receiver. For optimal usage maintain the antenna in a vertical orientation and be near a window.

Do any aircraft panels have a "connect handheld here" RF jack so you can use the plane's external antenna with an handheld radio? Seems like an option at least worth considering, though you'd also have to store the coax cable to use as a jumper. I know our portable backup radios on the ground are normally connected to external antennas but have portable whip antennas in case we need to bring them with during an evacuation.

@randomhead if you ask your mechanic to install one they might, nothing i have ever flown has had one.


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Is there a standard time zone for flight schedules?
I was reading a question on Travel.SE. The asker was asking whether the time on his ticket was in Daylight Savings Time or in standard time. I would imagine flight schedules are supposed to be time-zone/DST agnostic so as to avoid any potential confusion and timezone conversions.
A commenter mentioned UTC time, which makes sense to me.
So my question is: is there a standard time zone or other time standard used for tickets and/or flight schedules?

@Him Not really a duplicate, but indeed worth pointing the user in that direction to explain why Zulu is used in Aviation.

Moonman, also reference this: Does DST affect local airspace?.

I call time on my ticket as my flight schedule. Are you asking about flight schedule as seen from passenger or pilot?

Customer facing documents like boarding passes and itineraries typically show local times for ease of use.  Just use airport clocks.  I would not trust the flying public to compute UTC.  It would be chaos.
Crew facing documents will usually be set to UTC, or zulu time.


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When do airline pilots arrive at airport?
How much time it takes to prepare a flight, to control the aircraft, before departures?
How much spare time takes the pilots, to reach a foreign airport? I never experienced a delay because of missing pilots because of road traffic (or road accidents), but I assume this is accounted in working time, so airlines will not push pilots to arrive too early.

You'd be surprised how poorly some airlines treat their staff. Not sure about pilots but cabin crew shifts at many U.K. airlines aren't considered to have started until chocks off yet they have to be at the airport crew room an hour before takeoff.

"When do airline pilots arrive at airport?" At the same time as the airplane. :)

Most airline agents aren't going to announce that the flight is delayed because the crew got stuck in traffic, even if that's what happened. I have had flights delayed "due to crew availability," but that generally happens during bad weather and crews may have exceeded their duty times due to earlier delays.

@Notts90, most, if not all, airlines pay their crews by flight hours. But it can't really be considered a bad treatment—you are paid to do job and the flight hours are just used as proxy for measuring how much work you've done each month when you can take shifts irregularly. It does mean you won't get paid overtime for delays, but that's clearly intentional as it makes it everybody's best interest to be ready on time.

@Notts90, also, the duty definitely does start at the moment when you are required to arrive at the airport and this is important for determining when it must end for the day. It just does not affect the calculation of your salary for that day.

@JanHudec that's my gripe with such airlines, you're classed as on duty and at the airport but not getting paid. Not all airlines run that way, mainly the cheaper ones.

Airline pilots generally arrive at the airport 2 hours before departure.
Typically, a pilot will gather in a meeting with other pilots flying the same flight (who he/she may have never met before; but that is not a problem as there are Standard Operational Procedures which every pilot follow). They will discuss the flight plan (including expected delays, departure / arrival routes and procedures), weather, review the loading sheet & fuel etc.
After the pilots' meeting, they will also meet the flight crew and have a brief discussion about the flight. Matters such as expected delays, transport of patient (sick passengers) needing medical attention etc. will also be brought out.
Around 30~45 minutes before departure, the pilots will board the plane. One pilot will program the Flight Management Computer in the cockpit while the other will conduct a walk-around.
There are "stand-by" pilots ready to takeover whoever is unable to report to duty at the airport. They can be anywhere and doing anything while on call, however they must be able to arrive at the airport typically 1~2 hours after receiving a call. Also, they must not consume alcohol while on call as this will invalidate their flying status.

I once heard that pilots aren't allowed to drink anything 12(?) hours before the flight, I guess this is also true for the pilots on call?
Also, do pilots do a walk around the plane? Or is this done by somebody else?

@ROIMaison Pilots always do their own walk-around. Each of them will do it separately. Back when there were flight engineers all three of them would do it.

It probably depends on the airline. My girlfriend was a cook on Turkish Airlines flights, so I asked her about it to have a first data point.
Pilots
(note: she never asked a pilot about the times, but they were always there when she arrived, so these are her best guesses):

2 hours on narrowbody
2 hours on widebody

Cabin crew:

1 hour on narrowbody
1.5 hours on widebody

Cooks:

1 hour on narrowbody
2 hours on widebody

She (and the cabin crew) also had stand-by duties, where she had to be at the airport for a long time, just as a back up when somebody did not show up. A similar system was also present for pilots, but then from their home. I imagine that someone being there to fly the plane is much much much more important than a couple of pilot hours worth of fees.

No no, she was an actual cook on the airplane, it was an initiative of Turkish Airlines to provide a better service, I'll change it to cook.

Ah, interesting! I hadn’t heard of in-flight cooking before.

From my experience (back in the 90's) the pilots arrived in pecking order. The flight engineer was always there way ahead of the other two. Then the F/O about 30 min before pushback. Sometimes the captain arrived with the F/O and sometimes the captain would show up 15 min before pushback, do a walk around,  briefing, then go. I believe now the F/O is usually responsible for completing all the paperwork and setting the FMS.

Btw,  this was for cargo flights. Plus the F/O was responsible for briefing us jumpseaters. About the only time the captain and F/O spoke to non-pilot jumpseaters was when you brought out the cookies or the goldfish crackers you brought - it took me several trips before I realized that was pretty much expected When you're hitchhiking it pays to be nice :)


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Why is there only circumferential treading on the nose wheel?
The pilot uses the nose wheel for steering on the ground, still all the tires on the aircraft use the same treading. Why are there no treads perpendicular to the rotation direction?
I know that most of the wheels on an aircraft are there just for the ride since there is no propulsion system acting on the wheels like with a car but the nose wheel has a similar task as the front wheels of lets say a lorry.

I believe the answer primarily has to do with allowing a channel for water dispersion to mitigate the chance of hydroplaning. That is, the water is thereby allowed to move into the channel and forward and aft, instead of forming a "wedge" under the tire and inducing hydroplaning. The concept is very similar to the corresponding grooving in many runways.

Actually, there is a propulsion system acting on the wheels: Brakes

@sweber: If brakes are propulsive devices... could they be used to increase perpetual motion efficiency?


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What is the least expensive way to become a commercial pilot in the UK?
Besides the RAF is there a way of becoming a pilot without spending tens of thousands of pounds on flight training? I'm finishing university and looking to go into a career in flying, I can't join the RAF since I am partially colourblind so I would not be allowed to train as a pilot in the Air Force.

Marry a really rich person. There are additional benefits to that too.

This is tough to answer. The money has to come from somewhere and a lot depends on your situation, your goals and how serious you are about becoming a pilot. Some options are a training grant, a higher education grant, support from your family, selling your house, getting a bank loan, etc., possibly combined with moving to another country where training is cheaper.

I was more wondering if there are any programs done by airlines where you get tuition payed and then work the hours off or something like that

See this answer regarding British Airways.

Also, are you sure that the colorblindness wouldn't be a problem for becoming an airline pilot? At least in the U.S., a Class I medical certificate (required for airline transport pilots) has the requirement that the pilot have the "Ability to perceive those colors necessary for the safe performance of airman duties." If RAF considers your condition a no-go, then EASA might, too. If you can't easily tell red from white, for example, that's going to be a big problem. :)

@reirab in my experience, if something's a problem in the US it's even more of a problem in Europe. EASA is generally far more strict than the FAA when it comes to medical and other regulatory requirements.

It looks like British Airways may still offer their Future Pilots program, which Danny Beckett mentioned in this answer.
Another possible option would be to move to another (probably non-Western) country for a while. Southeast Asia in particular is experiencing a large amount of growth in airline traffic and seems to hire lots of American, Australian, and European pilots, so you might be more likely to find openings in a program there that would cover the cost. The Middle East might also be an option, given their current huge growth in airline traffic, though I'm not sure what their hiring policies are like.
Cathay Pacific is an example of such a program in Southeast Asia. Among their listed programs for hiring flight crew is their Cadet Pilot Programme. They hire people from all over and send them to a flight training school in Australia. I remember them even advertising this program on the plane the last time I flew with them (a couple of years ago.) Being a former British colony, someone from the U.K. would probably fit in well there. I think every pilot I had on my last trip with them was either American, British, or Australian. Seems like it would be a fun airline to fly for, too, if you were ok with moving to Hong Kong for at least whatever their commitment period is. With the vast majority of their flights being international on wide-bodies, you'd likely get to heavy jets much more quickly than you would in Europe or, especially, the U.S.
One additional thing to note, though, as I mentioned in my comment earlier, if you don't meet RAF's requirements due to your eyesight, you may also have trouble getting the necessary medical certificate for an Airline Transport Pilot license, especially in Europe or the U.S. It turns out the EASA's requirement's regarding color vision are pretty much identical to the ones I mentioned in my comment earlier regarding FAA standards. According to the UK CAA:

(a) Applicants shall be required to demonstrate the ability to perceive readily the colours that are necessary for the safe performance of duties.
(b) Examination
     (1) Applicants shall pass the Ishihara test for the initial issue of a medical certificate.
     (2) Applicants who fail to pass in the Ishihara test shall undergo further colour perception testing to establish whether they are colour safe.
(c) In the case of class 1 medical certificates, applicants shall have normal perception of colours or be colour safe.  Applicants who fail further colour perception testing shall be assessed as unfit. Applicants for a class 1 medical certificate shall be referred to the licensing authority.

There's a finite minimum set of requirements you must meet to get that ATPL and type ratings, and there's a finite minimum cost associated with that.
Nothing's going to change that. The only thing you can possibly, maybe, change is who is going to pay that money.

So don't fail any of the tests. The ONLY way to reduce the cost is to complete the training in as short a time as possible. Which means minimising flight hours, simulator hours, and retaking of courses. All of which boils down to passing every test first try.

And it will still take tens of thousands of pounds.

Going the Air Force route is also takes tens of thousands of pounds, except the air force (and thus other people, taxpayers) pick up part of the bill.
Same with every other scenario in which you're not paying everything yourself. Someone, somewhere, is paying for it.
Used to be (and maybe still is, but it's unlikely given the glut of unemployed pilots) some airlines would take on people and train them from scratch. But that's now rare. Most at the very least require you to have your PPL already, and many won't hire you unless you are qualified on at least some aircraft in their fleet.

Same with every other scenario in which you're not paying everything yourself. Someone, somewhere, is paying for it. Very valuable point to remember.

I understand that, I wasn't looking for someone to teach me to fly for free rather a subsidized program by an airline because hopefully they want to train pilots haha. Thanks for the advice on the tests though, I wonder how brave a bank would have to be to give me a loan like that

"Someone, somewhere is paying for it:" ...and will expect something for their money, which is why military pilot training comes with a service obligation.

In addition to the cost, the FAA is setting some pretty high bars for U.S. airline pilots which I would assume are being mirrored elsewhere. An FAA commercial pilot's license requires a minimum of only 250 hours total flight time including a minimum number of hours in various specific situations (night/adverse weather, >50mi from home airport, multi-engine or jet aircraft). To fly for a major airline, however, requires 1500 hours total flight time before they'll even let you in the cockpit, then another 1000 hours as FO before you can fly as Captain.

On the bright side, since the U.S. Armed Services went volunteer/career, they've worked much harder to retain their trained flight officers for at least the full 20 years, a strategy also mirrored by other nations, so you don't see nearly as many of those pilots transitioning to civilian work as you did 30-40 years ago with the mass HDs of servicemen after a major engagement.

@KeithS otoh the US military is rapidly scaling down, so a lot of skilled pilots are being RIFfed.


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What is the noise that the A320 makes on the ground?
I've noticed that planes in the A320 family make a sawing noise on reaching the gate. The link below may give you an idea of what I mean (although the A320 in the video is still taxiing). Is this a hydraulic system or a pressure unit at work?

@mins The question you link to does not mention the PTU operation on the ground and the reason for it.

@mins But does not ever mention that the whole point is single engine operation which is typical on ground. However, this question asks specifically for the barking while on ground. So maybe from the POV of a professional, things are clear, but to an enthusiast, the first question provides much better and clearer information.


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How do landing gear retracting into the wing affect performance when deployed?
On many aircraft with low wing, landing gear (either wheel, leg, or both) retracts into the wing. Thus, when deployed, a significant part of the lower surface of the wing is not as clean as when the landing gear is retracted. It must affect aerodynamics by adding drag, but may also affect lift as this modify airflow along the wing.
For the aircrafts whose landing gear or landing gear leg retracts underwing:

How does the landing gear affect aerodynamics when deployed and when deploying?
Is it different for aircraft whose landing gear do not retract into the wing?
Does this modify performance critical when taking off or landing such as stall speed, optimal climb speed?

there's a reason why the gear are affectionately known as the "bottom speed brakes"

There are two main geometrical characteristics that affect the airflow in the situation mentioned. From one side you have a cylinder with wheels at the end; on the other side during the transition you have an open cavity.
Let's talk about the first one. You actually have a cylinder flying perpendicular to the air speed. This is kind of canonical problem in fluid dynamics, there is a lot of literature about this problem, but to explain the effect I would like to take a simple example that I took from the website, the infinite 2D cylinder. For that, I include this picture of flow over a cylinder:

Note for fluid dynamics experts: This is not relevant if Reynolds Number (Re) is around 10,000, but for understanding the concept is good enough.
So, imagine that we are cutting landing gear perpendicular to its length and we visualize the flow.
What do we see?
Two main things....

Looking at the flow lines, they are almost not affected at 3-4 diameters of distance from the cylinder perpendicular to the flow direction there is a very local influence of the existence of the landing gear.

However we see that down the flow there is a big influence with the creation of chaotic structures. That is creating a big increase in drag downstream of the landing gear and any surface behind.

Basically... low influence on the red direction and significant influence on blue. Answering your question, influences the wing in a local area.

The influence anyhow is negative on the lift, as the flow is accelerated locally, reducing the static pressure below the wing, having a lift reduction, but is a local effect. This also have an influence on the wing lift distribution and a small effect on drag... lower than the one directly produced by the landing gear itself.
Now.... the cavities, they have a bigger influence, firstly because they have bigger extension parallel to the airplane surface (the landing gear goes perpendicular to the airplane the cavity goes along the surface). Secondly, their flow behaviour is different, goes in and outside the cavity. I found this web page very good on this, but basically the flow behaviour is like these image:

I found several flows similar to the second one, but depends on the airplane. You can see that flow will enter and go outside and will have a big influence.
Finally, the biggest influence is found when the airplane is deploying or retracting the landing gears when you have changing conditions, difficult to predict and affecting the airplane dynamics. Usually you can notice it when landing or taking off, when landing gear is deployed you fell significant vibration on the airplane.
Concerning your questions:

How does the landing gear affect aerodynamics when deployed and when deploying?

There is a significant drag increase, slightly the lift, but also creates some unsteady behaviour affecting overall airplane mechanics creating vibrations (anyhow, nothing not already taken into account).

Is it different for aircraft whose landing gear do not retract into the wing?

I understand that you refer to airplane with non retractable landing gear. The effect is similar but usually the configuration tries to avoid  having the landing gear close to the wing, as finally they don't need to retract them back.

Does this modify performance critical when taking off or landing such as stall speed, optimal climb speed?

Well, stall speed is usually defined by wing lift curve, and not so much affected by the landing gear. Usually the limitation is on the upper side due to separation.
Climb speed is basically engine thrust minus drag, the drag increases the climb speed is lower. That's why airplanes retract the landing gear as soon as possible.

Re "(the landing gear goes perpendicular to the airplane the cavity goes along the surface)"-- not always-- consider Curtis P-40, Vought F4U Corsair.  Re "Is it different for aircraft whose landing gear do not retract into the wing?
I understand that you refer to airplane with non retractable landing gear."-- no, probably was asking about aircraft whose landing gear retract into the fuselage -- e.g. high-wing Cessnas, B-47, B-52, A-7, F-8, F-16, F-15, etc--

There are a few things to think about here.
For one deployed gear basically acts as a speed brake, many new GA planes are working to allow gear extension at full cruise so the gear can double as a speed brake, notable Diamond has done this on their new twin, and talks about it in a few of the videos. So from that stand point they act like any other drag creating air break.
Now lets talk about air flow over the wing. While a wing generates lift as a whole item, the lift is distributed along the length of the wing as is the air flow. Most GA planes (and the war birds you have pictured) have the gear in a more inboard position. Deployed gear may create a bit of turbulent flow on the inner part of the wing but the rest of the wing still generates lift while the gear is down and interrupting air flow, possibly effecting lift.
As for speeds you have Vs0 which is your stall speed in landing configuration which could differ from your Vs but should be confirmed in your POH for each aircraft. I would think from a design stand point the engineers make sure that the wing generates enough lift for take off and landing and safe flight with the gear down but focus mainly on aerodynamics from a gear up perspective as that is the common setup of a plane. You should also note that the drag from the gear will affect climb performance so you will want to get that gear up quickly once you are flying.

The main effect of the gear is its drag increase and a pitch-down moment due to the low location of the drag. Since fixed gears usually are more aerodynamic than the struts of a retractable gear, these effects are less pronounced for fixed gears. Another moment change is possible with retraction in flight direction; then gear activation causes a small shift of the center of gravity.
The drag increase is highest during gear deployment because the open wheel wells create additional drag. Normally, the drag of the extended gear leg, wheel bogie and wheels is the biggest part, so the open wheel wells will contribute maybe 30% of the total. Precise values vary wildly, depending on gear location, retraction direction and gear size.
The effect of open landing gear doors on lift is marginal, possibly even beneficial. So in total they lower L/D without affecting minimum airspeed. The higher drag shifts the optimum speeds for minimum sink speed and best cruise to lower values. Note that maximum speed is limited while the gear moves and is down.
Compare the effect on lift to that of a split flap: This also creates a lot of drag and separated flow, but when you compare the flap effectivity (pdf!), it will give more maximum lift than an equal camber flap. By keeping the upper surface unchanged, a split flap will affect the upper side pressure distribution less and will allow it to contribute more lift.

In the end, lift is created by deflecting air downwards. If the flow is impeded on the lower side of the wing, this helps with this deflection more than it hinders it. However, this is not an efficient way to increase lift - the drag increase is significant, and for that reason modern landing gear doors stay closed when the gear is not moving, except for those which are blocked by the extended gear.
Usually, the gear doors stick out in flow direction, so their blocking effect is small and local. Thus, their effect on lift is small.
Spinning wheels will create a gyro effect which will make the aircraft react in funny ways if the pilot wants to make small adjustments.

Does your answer mean that there is no significant differences when the gear retract into the wing and when it retract elsewhere (as in many high-wing aircraft)?

@ManuH: Not on lift, no. Drag is lower when the gear is shorter, but the question focuses on the effect of the wheel wells.

For any who may be interested in trivia, the landing gear retraction moment change for a 747-400BCF is -1,207,400 LB-IN. And though it pains my sense of engineering elegance and makes me shake my head over our (U.S.) national foolishness, the value that Boeing shows to the right of the LB-IN value is -547,700 KG-IN.

For me the main part of this question is about the effect on wing lifting performance, not drag. I don't know the answer to this technically, but I could offer the following thoughts: The area taken by the wheels is a small percentage (perhaps 2-5% at a guess) of overall wing surface. Even on heavy aircraft, each square foot of wing only lifts a small amount of the overall weight of the aircraft. This is defined as an aircraft's wing loading see https://en.wikipedia.org/wiki/Wing_loading for more details. So even if putting the gear down totally destroyed the lift of that area of the wing (which it won't because its not like a hole in the wing - the wing is still there just not the optimal shape) it would only be like adding a few pounds in weight to the aircraft and having to fly at a slightly larger angle of attack as a result of that.
So my conclusion is that the biggest affect of putting the gear down on any aircraft, is drag, the effect on lift will be minimal, and stall speeds are usually provided in the POH in landing configuration in any case. So this is not something a pilot would normally think about. The biggest deal for the pilot is the effect on the trim due to the position of the drag and the additional power needed to overcome the additional drag.


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Why has the 777x been designed with folding wingtips?
I recently visited the Boeing factory outside Seattle, WA and noted in the distance a new 777x being built with, what looked like very large winglets.
However, on closer inspection they're not - the ends of the wings are designed to fold up.

Source: aviation today
I didn't get a chance to ask our tour guide, so I was wondering if anyone here can explain why this aircraft has been designed with folding wingtips. I'm presuming this is only for ground operations, and my guess is something to do with size - but are they really so big that they need to fold up? I would have imagined that other large wide body jetliners would have similar dimensions.
Please note that I'm not asking about whether this should have winglets, or if winglets provide any benefits, I'm asking specifically why the ends of these wings fold up rather than just fixed as per previous 777 models.

I think (which is why this isn't an answer) is that they need to fit into certain stands, otherwise they could use larger (more expensive) stands that have less density. Here is a video about the 777x folding wingtips.

Closely related and might even answer this?

Only way it would fit through the factory doors ;)

The Boeing 777X website states that this is to enable a more efficient wing (read: wider span) while maintaining the airport gate and taxi footprint of the classic 777 (which ensures airlines can use the 777X on roughly the same airports and intermix operations with the classic fleet).

Many modern aircraft have been designed with winglets, and older ones have been retrofitted with them. They allow a wing to produce more lift with less drag. However, the benefit is even greater if the wingspan is simply extended.
The downside of increasing wingspan is that wingspan affects many aspects of airport infrastructure, especially taxiways and gates. FAA Advisory Circular 150/5300-13 covers airport design, and includes an "Airplane Design Group" categorization, from I to VI. Group V has an upper cutoff of 214 feet (65 m) wingspan.
The 777-200LR and 777-300ER have the largest wingspan of the existing 777 family, at 212 ft 7 in. The 777X will have a wingspan of 212 ft 9 in when the wingtips are folded. This ensures that the 777X will be able to fit on every taxiway and into every gate that the current 777 can, while gaining the benefits of an increased wingspan in flight with the wingtips folded down.

Exactly. The folding wingtips allows the 777X to use stands and taxiways designed for ADG V aircraft, rather than needing ADG VI stands and taxiways like the A380 and 747-8. Even airports designed to accommodate A380s and 747-8s still typically have far more ADG V stands than ADG VI stands. ADG VI is basically just the 747-8 and A380, while ADG V is all 777s, 787s, A330s, A350s, and 747s prior to the -8. Bumping the 777X into ADG VI would have been a massive cost increase to operators, making the aircraft much less competitive.

But will the folded wingtips unfold in the air during flight or do they stay folded?

They would unfold on the ground when it's safe during taxi operations but before entering the runway. This ensures that the crew can confirm they are locked in place without running the risk of interfering in airport operations by taking up an active runway.

Folding them up reduces wing tip vortex. So, take of and landing planes can be stacked closer together without encountering wake turbulence.
Then fold them out in the cruise to benefit from the bigger wing area.

Folding them up reduces wing tip vortex this is the opposite of true. (nevermind that the wingtips don't fold or unfold in flight)


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Which standard is applicable to the electric power system in civilian US aircraft?
I am trying to close a gap in my answer to this question:
Are there general accuracy requirements for aircraft power supplies?
There is MIL-STD-704(F) Aircraft Electrical Power Characteristics for military aircraft.
Which standard is applicable to the electrical power system for civilian aircraft under the authority of the FAA?

The answer is not straightforward.  Power supplies used in commercial aircraft LRUs are required to meet RTCA DO-160G, sections: 16 Power Input, 17 Voltage Spike, and 18 Audio Frequency Conducted Susceptibility - Power Inputs.
The issue is this is what the consumers of aircraft power have to deal with.  It doesn't actually specify what the aircraft power distribution system has to provide. Though by inference, the power generation system must deliver power within the bounds of what the consumers can accept.
In my experience, aircraft OEMs design their power distribution system and then levy specific requirements on the power supply design of all consumers on the aircraft. While DO-160 covers the majority of the need, the OEM requirements will include additional requirements deemed necessary by their design and analysis. As a Boeing engineer explained to me, "The requirements are not to protect your power supply.  They're there to protect my power bus."
The reality is aircraft power is a small dynamic system and it is impacted by everything on the bus.  It has to be designed as a system unique to each aircraft. Since it's part of the aircraft, the aircraft rules come into play.  For air transport aircraft, that means 14 CFR Part 25. The applicable portions include 14 CFR 25.1310 and the Electrical Systems and Equipment subject section.
Additional guidance is available in Advisory Circular AC 25.1353-1a ELECTRICAL EQUIPMENT
AND INSTALLATIONS.

You are probably looking for RTCA DO-160G.
Note that there is no legal obligation to apply such engineering standards under the authority of the FAA. It is the choice of the manufacturer and the customer which standards shall apply.
Of course there are many reasons to apply well established standards, including but not limited to cost-savings during development, testing, and certification.
The FAA strongly encourages the use of RTCA/DO-160G through Advisory Circular no. 21-16G:

[...]
a. This advisory circular (AC) identifies RTCA Document No. (RTCA/DO)-160 versions D, E, F, and G, Environmental Conditions and Test Procedures for Airborne Equipment, dated July 27, 1997, December 20, 2005, December 6, 2007, and December 8, 2010, respectively, as containing acceptable environmental qualifications to show compliance with certain airworthiness requirements. The FAA strongly encourages the use of RTCA/DO-160G for new articles.
[...]
 c. This AC is not mandatory and does not constitute a regulation. This AC describes a means, but not the only means, to comply with airworthiness requirements.
[...]


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How do you know which fuel an aircraft uses?
Other than the decals on the plane, how would you know what fuel to use in an aircraft? Where is this limitation documented?
For some aircraft, does this limitation include some sort of substitute fuel for emergencies? For example, I have heard that if Jet A-1 is not available, some aircraft may have approved emergency fuels, such as AvGas.
Where would this information be found?

I've heard that if there's really no jet A1 , you could use avgas on a jet engine but with limitations. I don't know if what i heard is true.

Amirul is exactly right. For example, many PT6 powered aircraft have a provision to run on AvGas for a limited time. This can be considered an "emergency" provision if the aircraft is stuck in a location without approved Jet fuels. This is an excellent question worthy of a good answer.

@JonathanWalters thank you for enlightening us :)

A jet engine will run on just about anything it seems: https://aviation.stackexchange.com/a/13048/2294

I think this question should be reopened. The question has been quite clear all along, though I suspect some folks are simply not familiar with the various aircraft limitations and provisions for emergency fuel. This is an excellent question which deserves a meaningful answer.

@SnakeDoc: Does that includes firewood and/or coal? I suddenly realize I never saw stock of them in the cabin..

@mins, coal slurry, yes.

The TPE331 will run on LL as well.  I believe the limitation is 100hrs of LL time every 1000hrs but I'd need to check.  The pilot instructions, if I recall, were to notify maintenance and stay below 12,000' or some other low altitude.

For what it's worth, the placards that you casually propose we ignore in your question are actually legally required (in the US) for airworthiness. :)

@JonathanWalters: That's really surprising, thanks for mentioning!

@egid the placards are certainly required to be installed on the aircraft, but may not be definitive as limitations. A placard may state "Jet fuel only" when other types of fuel are in fact authorized in certain circumstances.

The actual fuel type limitations will be listed in at least two places: the aircraft type certification data sheet (TCDS), and the limitations section of the aircraft Flight Manual, Operating Handbook, or equivalent.
The fuel placards placed at the fuel filler ports might not list all approved fuels, or may state "Type X fuel only" when additional fuels may in fact be approved. These placards should be understood as a precaution against inadvertently fueling with an unintended or wrong type of fuel. In some cases ignoring these placards can have fatal consequences.
Some aircraft do indeed have approved emergency fuels. This information would also be found in the limitations listed above.
For example, the TCDS for the Beechcraft King Air B200 with PT6A series engines is lists the following for normal approved fuels:

Fuel
  JP-4, JP-5 (MIL-T-5624); JP-8 (MIL-T-83133); JET A, JET A-1, and
  JET B conforming to P&WC S.B. 1244 or ASTM SPEC. D1655; in addition for
  B200 and B200C Chinese No. 3 Jet Fuel. See NOTE 6 for emergency fuels

Note 6 in the TCDS reads as follows:

NOTE 6. Emergency use of MIL-G-5572:
  Grades 80/87, 91/98, 100/130, and 115/145 are permitted for a total time period not to exceed 150 hours time
  between engine overhauls. It is not necessary to purge the unused fuel from the system when switching fuel
  types.

Likewise, the EASA TCDS for the King Air B200 lists the same limitations.
The Flight Manual for the King Air B200 also lists these limitations, as applicable, as well as some additional altitude constraints when using emergency fuels due to electric boost pump requirements.

In contrast to the actual fuel limitations, the placard placed at the fuel port for the King Air B200 lists very little of the actual limitations:

Source: Beechcraft King Air 200 Maintenance Manual

As an example, the Cessna Caravan, with a PT6-114a engine can run on Jet A or other fuels, such as 100LL.  The information is not always on placards. The POH is normally where this is documented.  If a replacement engine has been installed, there will likely be a supplement with the STC which will cover alternate fuels and any limitations.
Pilots normally refer to POH and not maintenance manuals and type certification data. Therefore the POH or it's equivalent is the first place I would look.  Besides, when you need to use an alternate fuel, the volumes of manuals and type certification data may not be readily at hand.
Addendum #1
Some aircraft, such as recip singles may have a STC permitting use of mogas, as an example.  In these cases the STC will have an operations supplement for the POH or equivalent.  This was actually a fairly common alternative fuel 20 years or so ago.

For what it is worth, I have never talked to a caravan pilot, or operations group, that has admitted to using gasoline as an emergency fuel. Most commonly it is used when operating in Arctic regions. In my experience, it makes a difference when operating at temperatures below 15 or 20 degrees Fahrenheit. I asked this question a couple of years ago of mission Pilots and they told me that when they had fuel issues they would typically take truck diesel fuel and run it through filters and use it in the Caravan.

For a polar deployed airborne sensor, I have talked with two ops groups who both use avgas in their turbine fuel mix. The mix is situational, and is normally selected by the destination ground temperatures, and by how long they anticipate the plane will be on the ground.

You know what fuel to put in your (light) aircraft, the same way as you know what fuel to put in your car - you've done it before and you know to lift the right nozzle at the bowser! For larger aircraft/airliners, chances are there is a person whose job it is to refuel your aircraft, and they will likely know very well what fuel to use. If they don't they would ask, or look up the correct fuel from somebody who does know.
There are a few things to help you remember, such as placards next to the fuel cap

The pipework/shut off at the bowser might also have a placards/indications/color-coding.

A for emergency fuel, that is part of flight planning - you calculate how much fuel you expect to use and add an amount on for emergency/contingency.

Reminds me of the Bob Hoover story: http://www.squawkpoint.com/2014/01/criticism/

In the US (and I think other parts of the world as well) there is mostly one type of aviation fuel available for piston engines—100LL. Some engines run on regular auto gas if it does not have ethanol added. My Cherokee had an auto gas STC before I upgraded the engine, but I never say it at any airport and don’t know if it is even available in California. My sticker looks like this http://www.chiefaircraft.com/id-low.html. On the Cessna, we don’t have a sticker, just text saying "Use 100LL only".

Placards only tell half the story, check the flight manuals for the rest of it
The placards at the fuel port will indicate what the primary or preferred type of fuel is:

For spark-ignition reciprocating-engine aircraft, 100LL aviation gasoline is the primary certificate fuel, and that's what the factory placards will likely say.  Some aircraft may have AFM or STC provisions for the use of other types of gasoline (such as other grades of avgas, or oxygenate-free mogas), but this will likely not be included on the placards.
Aerodiesel (compression ignition reciprocating-engine) aircraft are typically fueled using Jet-A, and thus will have that on the placard.  Some may be modified to use standard road diesel or blends of road diesel and jet fuel -- the placards won't mention this, but the AFM will.
Smaller turbine (turboprop and jet) engines also are primarily fueled using Jet-A or a military equivalent fuel, and this will be on the placards.  They can also often burn aviation gasoline or avgas/jet mixes but under operating restrictions that will be mentioned in the AFM.
Large turbine engines will require jet fuel, and kerosene-based jet fuel at that -- they cannot run on wide-cut jet fuels such as Jet-B or JP-4, likely due to boiloff issues.  These aircraft take enough fuel, though, that the only fueling systems capable of handling them are dedicated to jet fuel anyway.

Fuel ports help, but aren't the total solution
Jet fuel dispensers are supposed to use a wide, rectangular nozzle that is too big to fit into an avgas plane's fueling ports, thus preventing all but the most determined from misfueling an avgas-only plane with jet fuel.  However, a few aircraft are retrofitted with turbine engines without changing the fueling ports, some older aircraft may have large diameter avgas ports due to the flows needed, and some turbine helicopters do not accept the standard jet fuel spout (J-spout or "Hoover spout").  Hence, some jet fuel dispensers are fitted with narrow "rogue" nozzles, creating the very misfueling hazard the J-spout is supposed to prevent.


text:
How do I have a gravel kit certified as an STC?
Any idea how one certifies a gravel kit for an aircraft? I am interested in possibly developing and certifying a gravel kit as an STC.
I am looking for FAA regulations that would be involved in such a project.

Are you wanting to do a one time field approved alteration for one certified aircraft, develop your own Supplemental Type Certificate alteration for certified aircraft, or something else entirely?

I'd contact a Flight Standards District office in your local area about this.

The FAA has a nice checklist here.
You should read through the FAA's GUIDE FOR OBTAINING A SUPPLEMENTAL TYPE CERTIFICATE. I assume you are a civilian but,

This advisory circular (AC) is a certification guide for obtaining a
  supplemental type certificate (STC). It is intended for civilian
  applicants only. We have written it to help designers, manufacturers
  and installers applying for STCs to better understand the STC process.
  The guidance in this AC is general in nature and intended to help an
  applicant gain a basic understanding of the STC process and our
  respective roles and responsibilities.

You will need to fill out this application form and submit it to the FAA ACO office in your area.


text:
How do pilots manage the speed during the approach?
Once the initial approach fix is reached, what are the different "speed stages" to the runway?
I often read about landing speed and approach speed. For example on this chart, Boeing indicates various approach speeds.
I don't understand from what point an aircraft is supposed to be at the approach speed. Is there a specific distance threshold that you cannot cross above the approach speed? What about for the landing speed?
Are there other key speeds than those two ones between the IAF and the runway?

Are you flying a simulator?  I ask noting Boeing and 'speed stages'. ?

In additional of the existing good answers: Boeing provides in their operations manual an overall sequence ("pattern") on how to do the approach, from a configuration perspective (including flaps, landing gear, etc). This look like this for a B747-400 in an ILS approach with autopilot (from FCOM page 131 -- Large document!). From the configuration, speed limits are imposed as explained in the answers.

The Boeing chart you reference appears to use for the approach speed column the Vref speed of the aircraft at its max landing weight in the landing configuration. This is the speed you want to be at or close to when you start the landing flare (the two 747 carriers I flew for used Vref+5).
Absent charted speed limits, controller instructions, or company policy, there is no specific distance threshold along the approach path that you cannot cross above that speed. The requirement is to be at the proper speed when you enter the landing flare.
However, if you were, say, flying the classic stabilized approach, you would be in the landing configuration at this speed from the FAF (final approach fix) to the landing flare. The FAF is typically around 5 miles from the runway.
Now, insofar as speed stages, the requirement is not one of specific distance points along your approach path (again absent charted limits, etc.), but rather what you have to do speed wise while changing from a clean configuration to the landing configuration. You're approaching the airport with the gear and flaps up. The critical element speed wise on the approach is the wing configuration.
Given the 747-200 speed of 150 kts in that chart, if you slow to that speed with the flaps up, you will stall. The minimum safe speed would be around 80 kts greater than that. Go to flaps 1 (which really doesn't put the trailing edge flaps at 1 degree but rather puts out the leading edge flaps or slats as some call them). Once they're out you can safely slow to Vref+60. At flaps 5 you can slow to Vref+40. At flaps 25 you'll be able to safely slow to the Vref speed in normal operating conditions.
Other aircraft have different flap settings and different speeds. The important thing to remember is that you're changing the shape of the wing to be able to fly more slowly. Where you will be along your approach path when you do this is typically not determined by distance per se, but by time. Do it too soon and you burn more fuel. Do it too late, and the problem becomes one of not being at Vref when you need to be.

@Vijay Assuming the wing is in the landing configuratuon, no, there's no requirement to fly faster than that unless you want to. Remember, though, that if the wing is not in the landing configuration for the specified Vref, you need to keep your speed above the minimum safe speed for the configuration the wing is in.

@Vijay When you start the flare, ideally you will be at Vref (or Vref+5 or whatever your company desires for a little extra padding). From the start of the flare until touchdown (usually but not always in the touchdown zone) your speed and rate of descent will be decreasing, especially the rate of descent as the majority of the descent rate should be gone when the wheels touch.

@Vijay I'm not qualified to answer https://aviation.stackexchange.com/questions/61898/do-modern-aircraft-automatically-manage-the-deceleration-to-vref?noredirect=1#comment161840_61898 as the 727s and 747s I flew did not have FMCs. I retired in 1999. Back then we typically disengaged the autopilot before the application of flaps and hand-flew the airplane from that point. I typically hand-flew from the top of descent since I enjoyed that. Some if our aircraft had auto-land, but the op specs of both the 747 operators I flew for specifically forbad doing that because auto-land was not maintained.

I'm going to answer your questions in reverse order.

Is there a specific distance threshold that you cannot cross above the approach speed?

The short and simple answer is yes, there is such a point. More precisely there is a point, such that if it is crossed above the approach speed (with some margins) a missed approach becomes mandatory. The missed approach is not necessarily performed exactly at that point, but a landing is no longer permitted.
In this day and age an airliner is nearly never flown in such a manner that there is a gradual deceleration all the way to landing. Where that point, known as the stabilization point, exactly is depends on the aircraft flown, the airline policies and what type of approach is flown.
What has forced the industry and regulators into defining such a point is the finding that many accidents have been caused by what is known as an "unstabilized approach": Wikipedia: Unstabilized Approach.
In really broad terms a few different ways to define this point:

1000 ft AGL in IMC (sometimes the same in VMC also)
500 ft AGL in VMC (sometimes 1000 ft in VMC also)
The Final Approach Fix (in non-decelerated non-precision approaches)
Some much lower altitude for circling approaches and visual circuits

These will vary depending on the airline and the regulatory authority, so there is no definite reference where you can find these from. There are also other criteria than the speed that must be fulfilled at the stabilization point. There are some sources you can refer to for examples:

Airbus
Stackexchange question

Once the initial approach fix is reached, what are the different "speed stages" to the runway?

Referring to the first section of my answer the initial approach fix is not critical in determining your speed profile. Rather you determine the point at which you must be stabilized at approach speed first and work back from there and in this way determine the point where you would first have to start slowing down and configuring the aircraft step by step (flaps, slats, landing gear).
In an ideal world you would want a continuous deceleration from the point you decide to start decelerating all the way until your stabilization point. Flaps would be extended as soon as the max allowed speed for the next configuration is reached and the landing gear in a sequence that usually comes with some recommendations from the aircraft manufacturer.
That means that the location of the point where you start your initial deceleration depends on many variables: aircraft type, aircraft weight, altitude, wind conditions etc. In addition to that there are factors that make the world less ideal, like air traffic control, procedure speed limits etc.


text:
What should a pilot do if they feel sleepy?
What should a pilot of any large commercial passenger airplane do if they feel sleepy?
What is the maximum time per day during which  the pilot may control the airplane ?

As far as the maximum time per day part of your question, what country/regulations are you asking about?

In the Desert Shield buildup in the early 90s, though the FAA's international regulation said you could not be scheduled for more than 12 hours of flying in any 24 hour period, the rule was ignored. I was a 747 captain flying freighters and, as a low-seniority captain, I had the Brussels down to Saudi run. Nine hours down, unload, and 9 hours back, so we often went over the 12 in 24. We got very tired, but we had no access to the Air Force's go-pills.(dextroamphetmine as I remember), and even if we did, we were at risk for a drug test.

In a three-man crew, it was quite common to have one person sleeping, and not all that unusual for two to fall asleep. Not legal, but practical.

There was an incident in 2012 where pilots declared PAN and requested automatic landing, because they both felt tired.

Many years ago ATC used an aircraft's SELCAL to awaken pilots that overflew, I think, San Francisco. That's from memory, but when I looked for it on Google, I ran across this more recent instance from India http://articles.timesofindia.indiatimes.com/2008-06-26/india/27760600_1_selcal-dubai-jaipur-mumbai-air-traffic

If a pilot is not rested enough to safely operate the aircraft, they should not fly. If a pilot is flying and falling asleep, they should switch out with another pilot or land.
Current FAA regulations for domestic flights generally limit pilots to eight hours of flight time during a 24-hour period.
For the Air Force, actual flight duty periods vary depending on the aircraft type and mission but we are required to get 12 hours of crew rest with the opportunity for 8 hours of uninterrupted rest prior to flying.
You can read the FAA's Fact Sheet on Pilot Flight Time, Rest, and Fatigue here.

I believe the eight hour limit is only for commercial operations.

It may have changed since I retired, but when I was flying I seem to remember that the 8 hour limitation was a domestic rule. The international rule was 12 hours back then.

I think the hours limits has to do with the size of the crew?  Like, you have to have a 3 man crew for a 12 hour shift (So you have backup).  But I really don't know, I just thought I read that somewhere...

The first thing a pilot should do if they feel sleepy is Check the CO sensor.  Drowsiness is the first sign of Carbon Monoxide Poisoning.  Checking for CO poisoning only takes a second, and can be easily mitigated and remedied.
NOTE:  Apparently a Pulse Oximeter will not indicate a CO poisoning problem.
Hopefully, it is not CO poisoning, and they just partied to long the night before.

Good point.  Here's the FAA's safety sheet on CO poisoning.  http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/CObroforweb.pdf

Please be aware that the standard pulse oximeter does NOT detect CO poisoning.  Evidently CO and O2 in the blood look the same to a pulse oximeter.

@SkipMiller:  I did not know that, but it seems very important!  Thanks.  (editing answer)

I've never seen a large or turbine powered aircraft with a CO sensor visible to the flight crew.

Generally CO poisoning is not limited to a tired feeling; it's symptoms are more feeling physically ill i.e. Nausea, headaches, dizziness, etc.


text:
How does downwash bend airflow down? How can wing-tip vortices reach the airflow in front of the wing?
Wing-tip vortices only start occurring when the pressure difference on the two sides of the wing starts. Wouldn't this mean that by the time the downwash from the vortices is acting on the airflow it would already have bent on to the surface of the wing and could not be pushed down any more?

In this picture you can see that the flow is already on the wing and it can not bend down any further because the wing is there.
How do the vortices change the direction of the relative airflow before it reaches the wing? The vortices only occur on the wing so they should not be able to do this.
EDIT:
When I say downwash I am talking about wing-tip vortices pushing the airflow down therefore reducing angle of attack.

Sorry about my bad drawing skills. It is hard to draw with a mouse

"How does downwash bend airflow down?" it's unclear to me what you are asking. the downwash IS the "bent" airflow.

What is bending the airflow down if the downwash is the bent down airflow

the wing is bending the airflow, that's its job.

When I say downwash I am talking about the downwash that is caused from the down stroke of a wingtip vortex. The downwash that bends airflow down more then normal and causes a wing to be flown at a high angle of attack to maintain height. I will edit my question

If you use a drawing program, you can add text in the drawing. Built-in ones are Paint for Windows, Preview.for MacOS, and there's a whole array of free apps available. Also, it is good practice to upvote answers that are useful to you. The best one is usually Accepted.

There is no bending from the wing and additional bending from wingtip vortices. Rather, there is bending and that is the vortex. See also How does an aircraft form wake turbulence? and maybe also Is induced drag not caused by tip vortices? and the links therein.

Try drawing with pen and paper and make a photo. Drawing with a mouse is a terrible idea.


text:
What if all passengers move from one side to the other?
Given the question What happens when all passengers jump at once?, I wonder what would happen if they all moved from side to side. If all the passengers moved from the front to the tail, or the other way around, there might be balance problems, but what if they moved from right to left (or vice versa)? I imagine the fuselage is not wide enough for this to seriously affect the aircraft's balance, but I would like confirmation for this assumption.
To have the greatest visible effect, the answer may focus on aircraft with relatively wide fuselages (for instance, A380, B747, B777). Maybe the effect is more visible for aircraft with smaller wingspans, thus, answers may also mention GA  aircraft with a large enough body-width-to-wingspan ratio (the opposite of a glider).

Anecdotally: When you start learning to fly there's an even left/right distribution of weight (you + instructor). The first time you solo there''s a definite bias to the left which needs balancing. You soon become accustomed.

I don't recall noticing anything different in the lateral balance when I "got rid of" the instructor. Peformance was notably better though, as I'd be taking off ~10% lighter :)

It is not only fuselage width to wingspan ratio but the position and size of the ailerons. In any case the fuel in the wings has a much more substantial impact on roll-trim.

You can get a significant rolling tendency if you shifted cargo or pax all to one side, but not enough to affect controllability; it would just be annoying.  This would apply to most airplanes.
Transport airplanes will have a lateral Center of Gravity range that has to be respected, along with a maximum fuel imbalance limit, to ensure controllability in some critical case.   In the CRJs, which are quite narrow, if everybody piled into the seats in one side, you (or the autopilot) would definitely notice it and have to correct with aileron, although it would be no problem trimming it out.  A really large fuel imbalance would be a much bigger problem however and might use up quite a bit of the available roll authority, so there are fuel imbalance limits in addition to lateral CG limits.
FAR 25 requires that the roll trim system provide the ability to trim out the rolling moment from max lateral CG offset in the most critical case - on takeoff, one engine inoperative, climbing at V2 (engine failure safety speed), with maximum allowable fuel imbalance. On the CRJs this uses up only about a third of the airplane's total roll authority (the RJ's roll trim just offsets the neutral point of the control circuit).

On the other hand, the CRJ, having tail-mounted engines, has much less engine-failure-induced rolling tendency than (say) a 737.

Not that much less, because the engines aren't really that much closer to the center line.  It still yaws fairly hard single engine and once the nose has moved 10 degrees or so it starts to roll hard and if you do nothing with the rudder you will be using most of your aileron to keep it from flipping over, while flying along all cockeyed.  Pilots learn this quickly on their first V1 cut in training (the sim models this very well) and often hit the ground with a wingtip on the first try if they are late or not aggressive enough with rudder.


text:
What does the PBY Catalina's APU look like?
I found some specifications of the PBY Catalina that mention an APU, but I did not find any other references to this APU. It seems  logical that such a plane has an APU, and that this APU is not like modern gas turbine that may be fitted on airliners.
What does the APU look like in this aircraft? Where is it located?

Page 15 of the owners manual has a picture of the APU:
https://books.google.nl/books?id=DxDDz5KSWdYC&printsec=frontcover#v=onepage&q&f=false

Why don't you post that as an answer?

The link you provided is for a Canadian PBY-5A which were called "Canso" in Canada. I flew these aircraft in water bombing operations during the early 1980's.
In your link it describes the 1955 installation of a 28v Homelite APU. It was installed on the left (port) side of the aircraft in the "living quarters" compartment behind the landing gear. The Homelite APU was powered by a 1 cylinder 2 stroke engine, and were often referred to as a "Putt Putt".  They were only used if the Main batteries were weak or discharged.

This is a photo of a Homelite APU from a B-17. They were also used on B-24s and other large WWII aircraft.  It is nearly identical to the ones we used in the civillian PBYs that I flew.

The original military PBY-5A APU installation looks a little different. It appears to be located on the starboard side in the central compartment near the landing gear. It would have been removed when the water bomb tanks were installed.